Chronic anti-phencyclidine monoclonal antibody therapy decreases phencyclidine-induced in utero fetal mortality in pregnant rats

Illicit drug use during pregnancy is a serious social and public health problem inflicting an array of deleterious effects on both mother and offspring. We investigated the hypothesis that a murine anti-phencyclidine (PCP) monoclonal antibody (mAb6B5; K(D)=1.3 nM) can safely protect mother and fetus from PCP-induced adverse health effects in pregnant rats. Pregnant Sprague-Dawley rats (n=4-5) were intravenously administered bolus injections of PCP (1mg/kg) on multiple days during pregnancy. They were also chronically treated with anti-PCP mAb6B5 at 45 mg/kg as a PCP antagonist. This dose provided one mAb-PCP binding site for every four PCP molecules. Therapeutic and safety study endpoints included pregnancy outcome (litter size, number of live vs. dead pups), maternal hemodynamic status and locomotor activity. Maternal hemodynamic changes (i.e., blood pressure and heart rate) and locomotor activity were measured in dams from gestation days 6-21 (one day antepartum) using a radiotelemetry-tracking device with a femoral arterial pressure catheter. This mAb6B5 treatment regimen significantly (p=0.008) reduced the number of PCP-induced in utero fetal deaths (odds ratio=3.2; 95%CI 1.3 to 7.9) and significantly (p<0.05) reduced acute PCP-induced maternal locomotor effects in the second trimester. Maternal hemodynamic responses to PCP were not significantly affected by mAb6B5 treatment. In conclusion, these data suggest that anti-PCP mAb treatments administered during pregnancy can safely protect a mother and her fetus(es) from PCP-related morbidity and mortality even when the mAb dose is too low to significantly prevent other PCP-induced maternal pharmacological effects.

Supplementation With Goldenseal (Hydrastis canadensis), but not Kava Kava (Piper methysticum), Inhibits Human CYP3A Activity In Vivo

The effects of goldenseal (Hydrastis canadensis) and kava kava (Piper methysticum) supplementation on human CYP3A activity were evaluated using midazolam (MDZ) as a phenotypic probe. Sixteen healthy volunteers were randomly assigned to receive either goldenseal or kava kava for 14 days. Each supplementation phase was followed by a 30-day washout period. MDZ (8 mg, per os) was administered before and after each phase, and pharmacokinetic parameters were determined using standard non-compartmental methods. Comparisons of pre- and post-supplementation MDZ pharmacokinetic parameters revealed significant inhibition of CYP3A by goldenseal (AUC(0-infinity), 107.9+/-43.3 vs 175.3+/-74.8 ng x h/ml; Cl/F/kg, 1.26+/-0.59 vs 0.81+/-0.45 l/h/kg; T(1/2), 2.01+/-0.42 vs 3.15+/-1.12 h; Cmax, 50.6+/-26.9 vs 71.2+/-50.5 ng/ml). MDZ disposition was not affected by kava kava supplementation. These findings suggest that significant herb-drug interactions may result from the concomitant ingestion of goldenseal and CYP3A substrates.

Intravenous (+)-methamphetamine causes complex dose-dependent physiologic changes in awake rats

Hemodynamic and temperature dose-response relationships were characterized in freely moving rats following i.v. (+)-methamphetamine administration to mimic the rapid onset of effects experienced by many human users. Rats received saline and (+)-methamphetamine in a repeated-measures, mixed-sequence design at 22+/-1 degrees C. Significantly greater blood pressure and heart rate elevations were observed after 1.0 and 3.0 mg/kg (+)-methamphetamine vs. 0.1 and 0.3 mg/kg. The time to peak hemodynamic values and the duration of effects were significantly greater after 3.0 mg/kg vs. the lower doses. The time to peak temperatures was significantly longer after 1.0 mg/kg vs. the lower doses. Following 3.0 mg/kg, all rats experienced temperature decreases before having elevated temperatures. The duration and magnitude of the delayed temperature elevations were significantly greater after 3.0 mg/kg vs. the lower doses. In conclusion, the (+)-methamphetamine-induced hemodynamic and temperature effects were not temporally synchronized, and the complex responses were not linearly related to dose.

The effect of rate of drug administration on the extent and time course of phencyclidine distribution in rat brain, testis, and serum

The goal of these studies was to examine the relationship between the rate of phencyclidine (PCP) administration and PCP tissue distribution. The time course of PCP distribution in serum, brain, and testis after rapid (i.v.) and slow (s.c.) administration was studied. Brain and serum PCP concentrations after an i.v. bolus dose (1 mg/kg at 900 microg/min) were highest at 30 s and decreased biphasically, with serum concentrations decreasing 30 times faster than brain concentrations during the early phase. Consequently, the brain-to-serum PCP concentration ratio increased from 8:1 at 30 s to 14:1 at 20 min before equilibrating at a ratio of 3:1 that remained constant from 1 to 8 h. In contrast, the testis-to-serum ratio increased slowly from 1:1 to 12:1 over 4 h, and then remained constant. In a separate group of animals, an s.c. infusion of PCP (18 mg/kg/day or 3.6 microg/min) produced a brain-to-serum ratio (6:1) that remained constant throughout the 96-h infusion. Testis-to-serum ratios increased from 4:1 at 1 h to 12:1 at 8 h and then remained constant for 96 h. Steady-state infusion of a pharmacologically inactive dose (2.5 mg/kg/day) produced a brain-to-serum ratio (3:1) that was significantly lower than the ratio (6:1) after infusion of the three pharmacologically active doses (10-25 mg/kg/day). The temporary high brain PCP concentrations and the dynamic disequilibrium between brain and serum concentrations after rapid i.v. administration could provide a better understanding of the preference of the human drug abuser for rapid rates (e.g., i.v. or smoking) of drug administration.

Disposition of methamphetamine and its metabolite amphetamine in brain and other tissues in rats after intravenous administration

These studies characterized the concentration-time profile of (+)-methamphetamine [(+)-METH] and its metabolite (+)-amphetamine [(+)-AMP] in the brain and five other tissues after (+)-METH administration. Male Sprague-Dawley rats received a pharmacologically active (+)-METH i.v. bolus dose (1.0 mg/kg) or a nonpharmacologically active s.c. infusion (20 h at 1.2 mg/kg/day). Tissues (n = 3 per time point) were collected for more than four elimination half-lives in the i.v. group, or at a single steady-state time point (20 h) in the s.c. group. Based on data from the area under the concentration-time curves after i.v. dosing, the rank order of (+)-METH tissue accumulation was kidney > spleen > brain > liver > heart > serum with terminal elimination half-life values ranging from 53 to 66 min. (+)-METH concentrations were highest at the first measured time point (2 min) in all tissues except the spleen, which peaked at 10 min. The brain-to-serum concentration ratio rose from 7:1 at 2 min to a peak of 13:1 at 20 min before equilibrating to a constant value of 8:1 at 2 h. Following s.c. (+)-METH dosing, the (+)-METH brain-to-serum concentration ratio was the same as the equilibrated ratio following i.v. dosing. (+)-AMP concentrations peaked at 20 min in all tissues before decaying with terminal elimination half-life values ranging from 68 to 75 min. Analysis of the area under the concentration-time curve molar amounts of (+)-AMP and (+)-METH showed that (+)-AMP accounted for approximately one-third of the drug tissue exposure over time. Thus, these data indicate the importance of both (+)-METH and (+)-AMP in pharmacological effects following i.v. (+)-METH administration.

Anti-phencyclidine monoclonal antibodies provide long-term reductions in brain phencyclidine concentrations during chronic phencyclidine administration in rats

These studies examined the hypothesis that a single large dose of monoclonal anti-phencyclidine (PCP) antibody could provide long-term reductions in brain PCP concentrations despite continuous PCP administration. PCP (18 mg/kg/day, s.c.) was infused to steady-state (24 h) and then a mole-equivalent dose of a short-acting anti-PCP antigen-binding fragment (Fab) or a long-acting anti-PCP IgG was administered i.v. The PCP infusion continued for up to 27 days, even though the binding capacity of the single dose of antibody used should have been saturated within the first day. At selected time points after antibody administration, brain, testis, and serum PCP concentrations were measured. Serum PCP concentrations rapidly increased approximately 100- and 300-fold after Fab or IgG administration, respectively. Based on the antibody-bound PCP concentrations in serum, the functional elimination half-life (t(1/2lambdaZ)) values for PCP-Fab and PCP-IgG complexes were 9.4 h and 15.4 days, respectively. Fab and IgG administration produced a complete removal of PCP from the brain within 15 min. Although brain PCP concentrations were significantly decreased for only 4 h in Fab-treated animals, IgG administration resulted in significant decreases in brain PCP concentrations lasting for at least 27 days. In contrast, testis PCP concentrations were not substantially affected by antibody administration, suggesting that redistribution of PCP from the testis is too slow to benefit from a limited dose of antibody. These results indicate that anti-PCP IgG can preferentially protect the brain for approximately 4 weeks after IgG administration, even when the antibody binding capacity should have been saturated with continuously administered PCP.

Spontaneous locomotor activity and pharmacokinetics of intravenous methamphetamine and its metabolite amphetamine in the rat

The purpose of these studies was to better understand the behavioral effects and pharmacokinetics of an i.v. bolus dose of (+)-methamphetamine [(+)-METH] in a rat model of (+)-METH abuse. We characterized the behavioral effects after increasing (+)-METH doses (0.1, 0.3, and 1.0 mg/kg) and the pharmacokinetics of (+)-METH (and its metabolite (+)-amphetamine [(+)-AMP)]) at the lowest and highest of these doses in adult male Sprague-Dawley rats. The doses and route of administration were selected to mimic aspects of human use on a dose/body weight basis. Although the 0.1 mg/kg dose did not cause statistically significant increases in locomotor activity compared with saline controls, the higher doses (0.3 and 1.0 mg/kg) caused statistically significant increases in locomotor activity (p <.05), which lasted for up to 3 h at the highest dose. After the 1.0 mg/kg dose, the volume of distribution at steady state was 9.0 liters/kg, the total clearance was 126 ml/min/kg, and the average distribution and elimination half-lives were 9.2 and 63.0 min, respectively. Because the pharmacokinetic values after the 0.1 mg/kg dose were not different from those after the 1.0 mg/kg dose, the pharmacokinetics of (+)-METH were considered to be independent of the dose over this 10-fold range. (+)-AMP serum concentrations after the 1.0 mg/kg dose peaked from 10 to 30 min, and exhibited a T(1/2lambdaz) of 98.5 min. The statistically longer T(1/2lambdaz) of (+)-AMP (p <.05) suggested that the (+)-AMP terminal elimination rate and not the (+)-AMP metabolic formation rate is the rate-limiting step in (+)-AMP elimination following i.v. (+)-METH dosing.

Modifications of blood volume alter the disposition of markers of blood volume, extracellular fluid, and total body water

Recirculatory pharmacokinetic models for indocyanine green (ICG), inulin, and antipyrine describe intravascular mixing and tissue distribution after i.v. administration. These models characterized physiologic marker disposition in four awake, splenectomized dogs while they were normovolemic, volume loaded (15% of estimated blood volume added as a starch solution), and mildly and moderately hypovolemic (15 and 30% of estimated blood volume removed). ICG-determined blood volumes increased 20% during volume loading and decreased 9 and 22% during mild and moderate hypovolemia. Dye (ICG) dilution cardiac output (CO) increased 31% during volume loading and decreased 27 and 38% during mild and moderate hypovolemia. ICG-defined central and fast peripheral intravascular circuits accommodated blood volume alterations and the fast peripheral circuit accommodated blood flow changes. Inulin-defined extracellular fluid volume contracted 14 and 21% during hypovolemia. Early inulin disposition changes reflected those of ICG. The ICG and inulin elimination clearances were unaffected by altered blood volume. Neither antipyrine-defined total body water volume nor antipyrine elimination clearance changed with altered blood volume. The fraction of CO not involved in drug distribution had a significant effect on the area under the antipyrine concentration-versus-time relationships (AUC) in the first minutes after drug administration. Hypovolemia increased the fraction of CO represented by nondistributive blood flow and increased the antipyrine AUC up to 60% because nondistributive blood flow did not change, despite decreased CO. Volume loading resulted in a smaller (less than 20%) antipyrine AUC decrease despite increased fast tissue distributive flow because nondistributive flow also increased with increased CO.

Pharmacokinetic mechanisms for obtaining high renal coelimination of phencyclidine and a monoclonal antiphencyclidine antigen-binding fragment of immunoglobulin G in the rat

Our purpose was to determine mechanisms and methods for significantly increasing the renal coelimination of phencyclidine (PCP) and an anti-PCP monoclonal antibody binding fragment (anti-PCP Fab). To accomplish this goal, we performed a series of experiments to examine the dose-dependence of Fab elimination, mechanisms for enhancing PCP and Fab urinary coelimination and the antigenicity of repeated Fab administration. The results showed that urinary elimination of PCP and anti-PCP Fab was linear over a 30-fold range of doses. Anti-PCP Fab serum pharmacokinetics were best described using bi- or tri-exponential curves with a terminal elimination half-life of approximately 8 hr. Nevertheless, under all experimental conditions the early, nonterminal phase(s) were responsible for the majority (60%) of intact Fab elimination, with only 40% of the Fab eliminated during the terminal phase. These data suggest that the early rapid decline in Fab serum concentrations was primarily due to passive filtration and excretion of intact Fab, and not due to extravascular distribution as previously described. In comparison of methods for enhancing renal coelimination of Fab and PCP, systemic alkalinization produced a significant increase in Fab urinary elimination, with 69% of the Fab dose and 41% of the PCP dose recovered intact in the urine. Finally, in studies of the antigenicity of Fab, repeated administration of Fab produced no significant immune response or renal impairment. Overall, these experiments suggest that careful attention to the physiological status of the kidney during early time periods is essential for maximum coelimination of Fab and bound chemicals.

Efficacy of promethazine suppositories dispensed to outpatient surgical patients

Postoperative nausea and vomiting frequently complicate outpatient anesthesia and surgery. The duration of treatment for this complication must occasionally extend beyond discharge from the hospital. In this study, we evaluated the commonly used anti-emetic promethazine for its efficacy in the post-discharge period. Adult outpatient surgical patients who had excessive postoperative nausea and vomiting in the recovery room, or who were at risk for postoperative nausea and vomiting following discharge were given two promethazine suppositories (25 mg) for home use. All patients were contacted by our recovery room nurses on the first business day after their surgery and questioned as to their use of the suppositories and, if used, their efficacy. We found that 55 percent of patients given promethazine suppositories for home use had nausea and vomiting in the post-discharge period. Of the patients given promethazine, 89 percent used the suppositories. All of these patients reported improvement in their symptoms following use of the suppositories. None reported adverse effects from the promethazine suppositories. In conclusion, we found promethazine suppositories to be an inexpensive and efficacious treatment for nausea and vomiting in adult outpatient surgical patients following discharge from the hospital. Side-effects were minimal, and our patients voiced no complaints about this mode of therapy. We recommend this therapy for treatment of nausea and vomiting after hospital discharge following adult outpatient surgery.

The effect of halothane on the recirculatory pharmacokinetics of physiologic markers

BACKGROUND

The cardiovascular effects of halothane are well recognized, but little is known of how this affects drug distribution. The effect of halothane anesthesia on physiologic factors that affect drug disposition from the moment of injection was investigated.

METHODS

The dispositions of markers of intravascular space and blood flow (indocyanine green), extracellular space and free water diffusion (inulin), and total body water and tissue perfusion (antipyrine) were determined in four purpose-bred coonhounds. The dogs were studied while awake and while anesthetized with 1%, 1.5%, and 2% halothane in a randomized order determined by a repeated measures Latin square experimental design. Marker dispositions were described by recirculatory pharmacokinetic models based on frequent early and less frequent later arterial blood samples. These models characterize the role of cardiac output and its distribution on drug disposition.

RESULTS

Halothane caused a significant and dose-dependent decrease in cardiac output. The disposition of antipyrine was most profoundly affected by halothane anesthesia, which increased both nondistributive intercompartmental clearance and volume while decreasing fast and slow tissue clearances and elimination clearance in a halothane dose-dependent manner.

CONCLUSIONS

Halothane-induced changes in blood flow to the compartments of the antipyrine recirculatory model were not proportional to changes in cardiac output. Halothane anesthesia significantly increased (to more than double) the area under the drug concentration versus time curve due to an increase in the apparent peripheral blood flow not involved in drug distribution, despite a dose-dependent cardiac output decrease. Recirculatory pharmacokinetic models include the best aspects of traditional compartmental and physiologic pharmacokinetic models while offering advantages over both.

A recirculatory model of the pulmonary uptake and pharmacokinetics of lidocaine based on analysis of arterial and mixed venous data from dogs

Pulmonary uptake of basic amine xenobiotics such as lidocaine may influence the onset of drug effect and ameliorate toxicity. To date, pharmacokinetic analysis of pulmonary drug uptake has been only semiquantitative and ill-suited for relating pharmacodynamics to pharmacokinetics or for estimating the time course of the fraction of drug dose residing in the lung during a single pass. We have developed recirculatory models in an experiment in which lidocaine was injected into the right atrium simultaneously with markers of intravascular space (indocyanine green) and total body water (antipyrine); this was followed by rapid arterial and mixed venous blood sampling. Such models are interpretable physiologically and are capable of characterizing the kinetics of the pulmonary uptake of lidocaine in addition to peripheral tissue distribution and elimination. The apparent pulmonary tissue volume of lidocaine (39 ml/kg) was nearly ninefold greater than that of antipyrine (4.5 ml/kg). The recirculatory model characterized both arterial and mixed venous data, but the latter data were not essential for estimating lidocaine's pulmonary disposition either before or after recirculation of drug was evident.

Recirculatory pharmacokinetic models of markers of blood, extracellular fluid and total body water administered concomitantly

Pharmacokinetic models were developed to describe the disposition of markers of extracellular fluid (inulin) and total body water (antipyrine) from the moment of injection to incorporate the intravascular mixing component, determined by a marker of intravascular space (indocyanine green, ICG). The simultaneous dispositions of these markers were characterized in four halothane-anesthetized dogs. After injection of ICG, [14C]-inulin, and antipyrine into the right atrium, femoral arterial blood samples were collected every 3 sec for 1 min and less frequently to 20 min for ICG and to 360 min for inulin and antipyrine. ICG and antipyrine concentrations were measured by high-performance liquid chromatography and [14C]-inulin concentrations were measured by liquid scintillation counting. The marker concentration histories were characterized completely by fully identifiable recirculatory compartmental models. Because neither ICG nor inulin distribute beyond intravascular space before recirculation, their first-pass data were modelled simultaneously to improve confidence in central circulation model parameters. This central circulation model included an estimate of cardiac output that was retained in the recirculatory models of all markers. Three tissue compartments were identified for antipyrine, a lipid soluble marker that equilibrates with tissue (including the lung) and estimates total body water and tissue blood flow. The hydrophilic marker, inulin, diffuses into interstitial fluid so slowly that only two extravascular compartments were identified. These models may be used to determine how cardiac output and its distribution, pulmonary drug uptake, and nondistributive blood flow contribute to variability in patient response to drugs with a rapid onset of effect.

Induction and maintenance of anesthesia in dogs by intravenous administration of methohexital

OBJECTIVE

To devise and test an i.v. methohexital infusion regimen for induction and maintenance of surgical anesthesia in dogs from which they would rapidly recover.

DESIGN

Dose-response and plasma concentration-effect study.

ANIMALS

11 clinically normal dogs.

PROCEDURE

Bolus methohexital pharmacokinetic variables were determined in ketamine- and pentobarbital-anesthetized dogs. Plasma methohexital concentrations required to inhibit purposeful movement in response to painful stimuli were determined during a stepped methohexital infusion in the same dogs on a second occasion. These pharmacokinetic/pharmacodynamic data were next used to design a bolus and two-stage infusion regimen that would result in stable plasma methohexital concentrations with prolonged infusion. This regimen was tested in a second group of dogs.

RESULTS

Mean steady-state volume of distribution of methohexital in the anesthetized dogs was 1.50 L/kg of body weight and mean elimination clearance was 10.2 ml/kg/min. Mean plasma concentrations required to prevent movement response to a noxious stimulus and at which the dogs could be extubated were 11.8 and 6.9 micrograms/ml, respectively. After a 6-hour infusion, recovery of airway reflexes sufficient to allow extubation required 67 minutes.

CONCLUSIONS

An easily implemented i.v. methohexital infusion regimen for induction and maintenance anesthesia in dogs was developed. During a 6-hour infusion, hemodynamic variables did not change. Use of this regimen resulted in anesthesia of sufficient depth to prevent withdrawal in response to noxious stimuli and in reliable and acceptable emergence times for use in canine survival studies in a cost-effective manner.

Effect of infusion rate on thiopental dose-response relationships. Assessment of a pharmacokinetic-pharmacodynamic model

BACKGROUND

The rate of administration of an intravenous anesthetic induction agent is an important variable determining the total dose required to reach a given endpoint, such as loss of consciousness (LOC). The influence of infusion rate on the dose-response relationship has not been described rigorously. In this study we characterized the effect of different thiopental infusion rates on the times and doses required to reach a clinical (induction) endpoint.

METHODS

Fifty-six healthy, non-premedicated men, aged 19-59 yr, were randomly assigned to receive one of seven different thiopental infusion rates (40, 60, 75, 150, 300, 600, and 1,200 mg/min). The infusion was continued until the patient dropped a held object, indicating LOC. The infusion rates were selected using a simulation which predicted the relationship between the rate of administration and cumulative dose administered at the time of LOC. Average population pharmacokinetic parameters from a three-compartment thiopental model were combined with an effect-site rate constant for thiopental equilibration of 0.58 min-1 and a median effect-site concentration of 13.8 mg/l from previously published pharmacokinetic and pharmacodynamic models for thiopental. This derived model was used to predict the total amount of thiopental required, at each infusion rate, to produce LOC.

RESULTS

The observed median effective doses for infusion rates of 40-150 mg/min were similar and ranged from 296 to 318 mg. Dose requirements increased significantly with increasing infusion rates greater than 150 mg/min; median effective doses for infusion rates of 300, 600, and 1,200 mg/min were significantly different from each other (436, 555, and 711 mg, respectively). The original simulation underestimated the observed thiopental doses at all but the lowest infusion rate. A new simulation was performed using a recently developed combined pharmacokinetic-pharmacodynamic model. This model incorporated a four-compartment thiopental pharmacokinetic model with quantal dose-response data to derive an effect-site rate constant for thiopental equilibration of 0.29 min-1 and a median effect-site concentration for LOC of 11.3 mg/l. The median thiopental doses predicted by this new simulation under the extreme conditions of a 30-fold range of infusion rates were within 13% of the observed doses.

CONCLUSIONS

In this study we quantified the relationship between the rate of thiopental administration and the resultant cumulative thiopental dose necessary to produce LOC. This study validated a novel pharmacokinetic-pharmacodynamic model based on a four-compartment pharmacokinetic model and infusion quantal dose-response data. Finally, we demonstrated that thiopental dose-response relationships are dependent on drug administration rate, and found that the ability to predict this dependence accurately is influenced by the pharmacokinetics, pharmacodynamics, and median effect-site concentration used to simulate the dose-response relationships.

Reevaluation of a maneuver to visualize the anterior larynx after intubation

Confirmation of translaryngeal placement of the tracheal tube can be unexpectedly difficult. This study examined the usefulness of displacing the larynx posteriorly with the tracheal tube during laryngoscopy with a straight laryngoscope blade to confirm tracheal tube placement. One hundred ASA Classes I, II, or III patients presenting for elective surgery whose normal anesthetic care included placement of an orotracheal tube via direct laryngoscopy were enrolled in this institutionally approved study after giving their written, informed consent. The view of the larynx at laryngoscopy was graded, and the tracheal tube was then inserted. When the larynx was incompletely exposed, the tracheal tube was displaced posteriorly while the laryngoscope was maintained in the intubating position in an attempt to better visualize the larynx. The effect of the maneuver on Mallampati grade for laryngeal exposure was noted. During laryngoscopy with a Miller blade none of the glottis was initially visualized in 17 patients (Mallampati laryngeal Grades 3 and 4). Thus, the tracheal tube actually was not seen to pass between these patients' vocal cords. Use of the maneuver resulted in improved visualization of the intubated larynx in 12 of these patients, and confirmed tracheal intubation. This maneuver is recommended as an aid to the anesthesiologist in the confirmation of tracheal intubation.

A pharmacokinetic-pharmacodynamic model for quantal responses with thiopental

The pharmacokinetic-pharmacodynamic model developed here characterizes the relationship between simulated plasma concentrations of thiopental and two dichotomous endpoints determined at induction of anesthesia: loss of voluntary motor power (clinical endpoint), and burst suppression of the electroencephalogram (EEG endpoint). The model incorporated data from two separate thiopental patient studies: a pharmacokinetic study with 21 males, and a pharmacodynamic study with 30 males. In the pharmacodynamic study, cumulative quantal dose-response curves for the clinical and EEG endpoints were developed from observations made during a constant-rate infusion of thiopental. Population mean parameters, derived from the bolus pharmacokinetic thiopental study, were used to simulate concentration-time data for the 150 mg.min-1 thiopental infusion rate used in the dose-response study. A single biophase model incorporating the two endpoints was generated, combining the pharmacokinetic and pharmacodynamic data from the two groups. Estimates of the mean effective thiopental concentrations affecting 50% of the population (EC50S) for the clinical and EEG endpoints were 11.3 and 33.9 micrograms.ml-1, respectively. The half-time for equilibration between arterial thiopental and the effect compartment was 2.6 min. These results are in reasonable agreement with previously described quantal concentration-response data, and with pharmacodynamic models developed for graded EEG responses. Simulation of bolus doses of thiopental with the new model provided ED50s for the clinical and EEG endpoints of 265 mg and 796 mg, respectively; the dose predicted to produce loss of voluntary motor power in 90% of an adult male population was 403 mg. A model combining population pharmacokinetics with cumulative dose-response relationships could prove useful in predicting dosage regimens for those drugs with responses that are categorical.